Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as described herein can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a mask pattern is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant. McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
The photolithographic masks referred to above comprise geometric patterns corresponding to the circuit components to be integrated onto a silicon wafer. The patterns used to create such masks are generated utilizing CAD (computer-aided design) programs, this process often being referred to as EDA (electronic design automation). Most CAD programs follow a set of predetermined design rules in order to create functional masks. These rules are set by processing and design limitations. For example, design rules define the space tolerance between circuit devices (such as gates, capacitors, etc.) or interconnect lines, so as to ensure that the circuit devices or lines do not interact with one another in an undesirable way. The design rule limitations are typically referred to as “critical dimensions” (CD). A critical dimension of a circuit can be defined as the smallest width of a line or the smallest space between two lines. Thus, the CD determines the overall size and density of the designed circuit.
Of course, one of the goals in integrated circuit fabrication is to faithfully reproduce the original circuit design on the wafer (via the mask). Another goal is to use as much of the semiconductor wafer real estate as possible. As the size of an integrated circuit is reduced and its density increases, however, the CD of its corresponding mask pattern approaches the resolution limit of the optical exposure tool. The resolution for an exposure tool is defined as the minimum feature that the exposure tool can repeatedly expose on the wafer. The resolution value of present exposure equipment often constrains the CD for many advanced IC circuit designs.
As the critical dimensions of the circuit layout become smaller and approach the resolution value of the exposure tool, the correspondence between the mask pattern and the actual circuit pattern developed on the photoresist layer can be significantly reduced. The degree and amount of differences in the mask and actual circuit patterns depends on the proximity of the circuit features to one another. Accordingly, pattern transference problems are referred to as “proximity effects.”
To help overcome the significant problem of proximity effects, a number of techniques are used to add sub-lithographic features to mask patterns. Sub-lithographic features have dimensions less than the resolution of the exposure tool, and therefore do not transfer to the photoresist layer. Instead, sub-lithographic features interact with the original mask pattern and compensate for proximity effects, thereby improving the final transferred circuit pattern.
Examples of such sub-lithographic features are scattering bars and anti-scattering bars, such as disclosed in U.S. Pat. No. 5,821,014 (incorporated herein by reference), which are added to mask patterns to reduce differences between features within a mask pattern caused by proximity effects. More specifically, sub-resolution assist features, or scattering bars, have been used as a means to correct for optical proximity effects and have been shown to be effective for increasing the overall process window (i.e. the ability to consistently print features having a specified CD regardless of whether or not the features are isolated or densely packed relative to adjacent features). As set forth in the '014 patent, generally speaking, the optical proximity correction occurs by improving the depth of focus for the less dense to isolated features by placing scattering bars near these features. The scattering bars function to change the effective pattern density (of the isolated or less dense features) to be more dense, thereby negating the undesirable proximity effects associated with printing of isolated or less dense features. It is important, however, that the scattering bars themselves do not print on the wafer. Thus, this requires that the size of the scattering bars must be maintained below the resolution capability of the imaging system.
Accordingly, as the limits of optical lithography are being enhanced far into the sub-wavelength capability, assist features, such as scattering bars, must be made smaller and smaller so that the assist features remain below the resolution capability of the imaging system. However, as imaging systems move to smaller wavelengths and higher numerical apertures, the ability to manufacture the photomasks with sub-resolution scattering bars sufficiently small becomes a critical issue and a serious problem. Additionally, as the size of the scattering bars decrease, their ability to reduce proximity effects is reduced. Some of the problems arising with the use of scatter bars are due to the fact that the ability to include scatter bars in a design is limited, for example, by the space dimension between adjacent main features.
In addition, it has been determined by the inventor of the present application that benefits arise when assist features are positioned at frequencies that coincide with the harmonics of the frequency of the primary mask features. This implies that the assist features should be positioned at integer multiplies of the main feature frequency. For a single scatter bar solution, the scatter bar is typically placed midway between the main features. However, such placement presents a problem for dense features because the scatter bar cannot be made small enough to avoid printing. Furthermore, for semi-isolated features, the impact of a single scatter bar is generally insufficient, thereby requiring that multiple scatter bars be positioned in the space region between main features. In such instances, the desirable frequency solution is not practical as the separation between the outer scatter bars and the main feature is too small.
Accordingly, the typical solution is to place multiple scatter bars equally spaced within the space between main features. However, there are problems associated with such a solution. For example, the resulting frequency of the scatter bars is often beyond the imaging limits, thereby eliminating all but the effect of the zero diffraction order of the scatter bar. Further, if the scatter bars are placed at a frequency that matches that of the dense main features, there is an undesirable increase in the likelihood that the scatter bars will print when utilizing modified illumination techniques.
More specifically, as shown in FIG. 1, which is an illustration of the use of prior art scatter bars as an OPC assist feature, when multiple scatter bars 11 are placed within a space opening between main features 10, the frequency of the scatter bars is a function of the main feature space width rather than of the pitch. Although the scatter bar frequency does not coincide with that of the main features, it is generally beyond the diffraction limits of the imaging system. As a result, no first order diffraction energy is collected from the scatter bars, making the scatter bar frequency inconsequential. For example, referring to FIG. 1, which illustrates 150 nm main features with a line:space duty ratio of 1:5, using a 248 nm wavelength and a 0.70 NA objective lens, a scatter bar size of 60 nm placed on a 187.5 nm pitch will result in three evenly spaced scatter bars 11 between the main features 10. The resulting k1 for the scatter bars, as defined by the resolution equation (k1=pNA/2λ, where p equals pitch, NA equals the numerical aperture of the objective lens, and λ equals the exposure wavelength), equals 0.27, thereby effectively eliminating lens capture of the first diffraction orders when using σ values of 0.95 and below. With only zero diffraction order collection, the entire space between the main features experiences a reduction in intensity as a function of the bar width (b) and bar pitch (pb):Space Reduction Intensity=[(pb−b)/pb]2=(0.68)2=0.46  (1)This result is expected if the space transmission is equivalently reduced. Diffraction energy of the main features is reduced, resulting in an image with lower intensity which can allow for a mask exposure dose that more closely matches that of the other features on the mask. However, control of the intensity reduction at other values is difficult with multiple scatter bar assist features due to the constraints of the bar width and pitch values of the scatter bars.
Thus, there exists a need for a method of providing OPC assist features in a photomask which eliminates the foregoing problems associated with the use of multiple scatter bar OPC assist features.